There are inherent problems with coupling a linear array of laser diodes with a solid state laser rod due to the geometry involved, and the properties of a typical Nd:Yag laser rod. Typically, a laser rod is side pumped by a linear array of laser diodes aligned in a straight line along the height dimension and oriented at the periphery thereof. As the laser diodes are energized, light enters through the side of the rod and the energy is absorbed by the atoms in the rod and excited or pumped to a higher energy level, thereby creating the lasing effect. Additional power output can be obtained from the rod by increasing the number of linear arrays mounted along the periphery thereof. However, these prior art arrangements have produced generally less than satisfactory results for several reasons.
One of the problems is that for smaller diameter rods, the optical thickness of the rod is minimized, thereby decreasing the absorption length or light path for the light emitted from the diodes. This results in only a small amount of pump energy being deposited within the lasing mode volume in the rod. A secondary effect caused by the poor rod absorption is the increased sensitivity to laser diode junction temperature and resulting wavelength mismatch experienced in the smaller diameter rods. Wavelength mismatch is the difference between the laser diode wavelength and the laser rod pump absorption bands. As the laser diode wavelength output varies with respect to temperature, performance can be seriously affected should the laser diode temperature not be carefully controlled to produce an output at the wavelength required to optimize absorption in the rod.
Several attempts have been made to improve the typical prior art arrangement to increase the coupling between the pump and the rod. One of these has been the addition of a sleeve over the rod, with a reflective coating being applied to the periphery of the sleeve except for a polished transparent flat or slit against which is mounted the linear laser diode array. In this configuration, laser diode light is directed through the sleeve, into the rod, through the center of the rod to the far periphery of the sleeve, and then reflected back through the center of the rod a second time. While this arrangement does offer some improvement, it still suffers from poor coupling efficiency in that nearly all of the pump energy is not concentrated in the laser rod nor in the laser mode volume within the rod. Furthermore, the only diode light that is reflected back through the rod must originate along a path through the center of the rod, and the optical design of this configuration does not permit most of the pump light to pass near the center of the laser rod.
To solve these and other problems in the prior art, the inventors herein have succeeded in developing a laser diode pump coupler which dramatically increases the efficiency of coupling between a linear array of laser diodes and a laser rod. The laser diode pump coupler of the present invention includes a cylindrical sleeve which slides over the laser rod, the sleeve having a reflective coating around its periphery with one or more transparent flats against which is positioned a pair of optical elements. The sleeve may either be hollow or solid, and if hollow, filled with transparent coolant fluid. If solid, the gap between the rod and sleeve is filled with an index matching compound, such as RTV.
The first of these two optical elements is a cylindrical focus lens for focusing the light being emitted by the linear array, and the second of these is a retro-reflector, the retro-reflector being mounted adjacent to the lens. Each lens focuses the laser diode beam from the linear array through the rod in an off-center alignment. By doing so, the beam impinges on the opposite wall of the cylindrical sleeve and bounces back through the laser rod in another off-center alignment to a point on the cylindrical sleeve's periphery spaced from its point of origin. As the sleeve is cylindrical, these reflections continue to traverse the rod in different paths. Thus, the beam continues to reflect back and forth through the laser rod thereby "walking" its way around the periphery of the sleeve.
Depending upon how many sets of focal lenses, retro-reflectors, and linear arrays are used, the beam will bounce through a corresponding angular sector of the sleeve and rod but eventually impinge upon a retro-reflector. The retro-reflector is designed to reverse the "walking" of the beam so that it substantially retraces its various paths through the laser rod until the pump light escapes through the focal lens at its point of origin. As can be appreciated, this pump coupler design has the advantage of theoretically multiplying the effective optical thickness (i.e. diameter) of the laser rod by 18-20 times to thereby concomitantly increase the pump light absorption length of the laser rod. Furthermore, because of the off-center focus, the pump coupler concentrates and focuses the pump energy within the laser mode within the laser rod resulting in maximum energy utilization between the laser mode and pump light. By increasing the effective optical thickness of the laser rod, the laser diode wavelength mismatch and temperature sensitivity is dramatically reduced, thereby providing a much wider range of wavelengths which can be selected for use as a laser pump, and also a wider range of temperatures over which the laser diodes may be operated with very small changes in laser output power.
Because of the very nature of the invention, the pump coupler design can be easily modified to increase the number of laser diodes either around the laser rod or along the laser rod by respectively adding more linear laser diode arrays or more diodes per array. In an alternative embodiment, offset reflective cylindrical elements may be utilized in place of the refractive cylindrical lens elements to collect and focus the laser diode light into the sleeve and through the laser rod. This provides flexibility and opportunities for design choice heretofore unavailable in the prior art. As their preferred embodiment, the inventors herein utilize three linear arrays with three associated sets of cylindrical focus lenses and cylindrical retro-reflector elements bonded to three flats located 120.degree. apart on the cylindrical sleeve. With this arrangement, there are three sets of beams which are independently "walking" their way around the periphery of the sleeve and which impinge upon and energize different portions of the laser rod.